A presence sensor is a sensor for remotely sensing the presence of a being or other object, typically a person. For example the sensing of presence may comprise detecting whether or not the being or object is in motion, determining a measure of its motion (e.g. speed or velocity), determining its range from the sensor, and/or simply detecting the fact of its existence in a certain zone (e.g. defined by the range of the sensor).
Presence sensors come in two kinds: active or passive. An active presence sensor is one which transmits a probing waveform and then uses a reflection of that waveform received back at the sensor to detect presence, e.g. using certain signal characteristics in the reflected signal to determine presence-related information. Examples of active sensors include ultrasonic, radio (e.g. radar) and microwave sensors which emit pulses of ultrasonic, radio frequency (RF) or microwave radiation respectively and sense presence based on the reflection thereof. A passive presence sensor on the other hand does not transmit its own waveform but instead relies on radiation emitted by the being or object to be sensed, or reflected from it but originating from another source other than the sensor itself. Examples of passive sensors include infrared sensors which sense the infrared radiation given off by an object such as a person or other being, and light sensors which detect light reflected from the being or object being sensed (e.g. originating from the sun or electric lighting in a room).
Presence sensors have a wide range of applications. One such application is found in controlling a lighting system. For example office lighting typically constitutes almost 30% of the electrical consumption in office buildings. It is known that light control strategies based on occupant presence information can be the most effective in reducing energy consumption. Hence the design of green buildings typically involves presence-adaptive lighting control systems.
Active sensors like ultrasound sensors tend to provide better detection than passive infrared sensors in large volumetric spaces. In larger physical areas, it is commonplace to have multiple active presence sensors for proper detection coverage. When such presence sensing systems are deployed however, there is a potential for cross-interference across active transmissions. For the system as a whole to function properly, ideally each individual presence sensor should be able to determine presence-related information in its respective coverage area. Cross-interference across active sensors (e.g. ultrasound or RF) is a commonly encountered problem in indoor and outdoor sensing applications, which can affect proper operation of the sensing and control systems. For instance, a sensor may trigger a false alarm when an adjacent zone is occupied based on an overheard transmission from a neighboring sensor. Such sensing malfunction can disrupt the lighting control system as a whole.
For example, in WO 2012/023087 the problem of frequency misalignment is recognized to cause detection performance degradation when attempting to detect moving objects in continuous-wave Doppler radars. For radar units that transmit at nominally the same frequency, slight frequency differences will typically occur due to tolerances of elements in the frequency determining circuits. If a frequency difference between two neighboring radar units is comparable with the Doppler shift produced by a moving object, a radar unit may mistakenly detect the transmission signal of another radar unit as being reflection from a moving object. The problem is addressed by choosing different frequencies of operation for different radar units, so that any given radar unit transmits at a frequency that is outside the detection frequency range of any other radar unit in the system, and therefore frequency misalignment does not result in cross-interference. However, a frequency allocation such as described in WO 2012/023087 may not in itself be sufficient in all possible sensing applications. For example it may not be sufficient in an ultrasonic sensing system due to the narrowband frequency response range of the transmitter.
Active presence sensors may also be found in other applications where the problem of cross-interference may occur, or more generally where operation of one active sensor may affect one or more other sensors.